Email gateway configuration when main email server is outside of firewall

Linux 2.4.8-26mdk
perl5 revision 5.0 version 6 subversion 1
apache 1.3.23

Does anyone have there main email server outside of the firewall. I am
having trouble configuring a way to create tickets via email. My RT is on
a machine within the firewall, and I have been trying to figure out how to
have email forwarded from the outside email server, through the firewall,
and into RT. Does anyone have this configuration, or any suggestions?

Munchy